A wide variety of technologies exist for removing a component or components from a mixture that also includes other components. These include distillation, gas absorption, rectification, stripping, regeneration, solvent extraction, etc. In each case, the traditional technology involves the use of vessels, example columns, to effect separation. Specifically, in the cases of distillation, gas absorption, stripping, rectification, regeneration, etc., the columns contain column internals such as packing or trays which are devices to provide the surface area for contact between two phases to cause the separation of components. This physical contact area separates the liquid flow into droplets which allows the gas to have a bigger area of intimate contact with the liquid. The performance of the device used to provide such surface area of contact is evaluated on such physical basis as the surface area per unit volume, wettability, pressure across the vessel, etc.
It has been suggested that, in the case of distillation, a chemical contribution (e.g. a catalyst) may be used in addition to the physical contribution. However, this strategy has not been applied in the removal of a component or components from a multi-component gas stream. Examples of such multi-component gas streams are combustion flue gases, natural gases, reformate gases, refinery gas, off gases from cement manufacturing, steel making, and the like. In these examples, the components that can be removed include, for example, CO2, SO2, SO3, H2S, and/or NH3.
In the case of combustion flue gases, refinery off gas, and reformate gas, it is known that the production and use of fossil fuels contribute to an increase in emissions of greenhouse gases (GHGs), especially carbon dioxide (CO2) and other pollutants such as oxides of sulfur (SOx), oxides of nitrogen (NOx), hydrogen sulphide (H2S) and hydrogen chloride (HCl). It is desirable to reduce the emissions of CO2 or the other pollutants. Large sources of CO2 emissions such as coal-fired power plants, refineries, cement manufacturing and the like are targeted to achieve these reductions. Thus, intensive research efforts have been made in recent years to develop methods for recovering the CO2 emitted from gas streams from these huge industrial emitters, and for storing the recovered CO2 without discharging it into the atmosphere.
One method of CO2 capture is gas absorption using aqueous amine solutions or ammonia solutions. Typically this method of gas separation technology is used to absorb CO2 from low-pressure streams such as flue gases emitted from power plants. An example of an amine used in this type of process is monoethanolamine (MEA). From a molecular structural standpoint, one of the advantages of using amines is that they contain at least one hydroxyl group, which helps to reduce vapor pressure and thus minimize the losses of the product during hot amine regeneration or CO2 stripping from the amine. Another advantage of using amines is that the presence of the hydroxyl group increases the solubility of the amines in aqueous solutions, thus allowing the use of highly concentrated absorbing solutions. Yet another advantage of using amines is that the presence of the amino group provides the necessary alkalinity to absorb CO2 (Kohl, A. L. and Reisenfeld, F. C., Gas Purification, 4th ed., Gulf Publishing Co., Houston, Tex., 1985; Kohl, A. L. and Nielsen, R. B., Gas Purification, 5th ed., Gulf Publishing Co., Houston, Tex., 1997). Thus, amines and ammonia have been the solvent of choice for CO2 removal on a commercial scale. In particular, aqueous amine solutions are the widely used solvents for CO2 and H2S absorption.
For many years, the amine process or the ammonia process for CO2 capture remained unchanged but recently demands to reduce energy consumption, decrease solvent losses, and improve air and water qualities have resulted in several modifications being introduced. For example, in the case of the amine process, specially formulated solvents have been introduced. Depending on the process requirements, for example, selective removal of H2S and/or CO2-bulk removal, several options for amine-based treating solvents with varying compositions are available. Also, improvements involving the overall integration and optimization of the plant configuration have been suggested. For example, U.S. Pat. No. 6,800,120 (Won et al.) describes a process configuration has been developed that allows the reduction of the heat duty for regeneration. Other improvements on CO2 capture technologies have been highlighted (Yagi et al., Mitsubishi Heavy Industries, GHGT7, Vancouver, 2004) based on solvent improvement, and special design of certain process units. CA 2,685,923 (Gelowitz et al.) describes a number of process configurations as well as a different amine formulation, the combination of which is said to achieve reductions in the heat duty for regeneration.
In a typical system, CO2 capture by absorption using chemical liquid absorbent involves absorbing CO2 from the flue gas stream into the absorbent flowing down from the top of the absorber column counter-currently with the flue gas stream, which flows upwards from the bottom of the column. The CO2 rich liquid from the absorber column is then pumped through the lean/rich exchanger to the top of the stripper column where CO2 is stripped off the liquid by application of steam through a reboiler thereby regenerating the liquid absorbent. The chemical absorption of CO2 into the liquid absorbent in the absorber is exothermic. The stripping of CO2 from the liquid absorbent in the stripper is endothermic and requires external heating. Typically the lowest temperature in the absorber column is around 60° C., which is limited by the temperatures of the lean liquid absorbent and flue gas stream temperatures, and the highest temperature is around 90° C. The typical temperature for stripping or desorption is in the range of 105° C.-150° C. The CO2 desorption process is endothermic with a much higher heat demand than the absorption process can provide thus setting up a temperature mismatch between the absorber and regenerator/stripper. This is one of the reasons that a large amount of external energy is required to induce CO2 stripping in the desorption tower. Since CO2 stripping is part of the CO2 capture process that employs chemical absorption, minimizing this external heat supply would be advantageous.
There has been interest in estimating the heat of chemical absorption of CO2 into liquid absorbents and the heat duty for stripping CO2 from the liquid absorbents for absorbent regeneration. Mechanistic verification would allow modifications to be designed aimed lowering the energy required for activation (e.g. Silva, E. F., Svendsen, H. F., 2006. Study of the Carbamate Stability of Amines using ab initio Methods and Free Energy Perturbations. Ind. Eng. Chem. Res. 45, 2497; Silva, E. F., Svendsen, H. F., 2007. Computational chemistry studies of reactions equilibrium of kinetics of CO2 absorption. International Journal of Greenhouse Gas Control I, 151; Jamal, A., Meisen, A., Lim, C. J., 2006. Kinetics of carbon dioxide absorption and desorption in aqueous alkanolamine solutions using a novel hemispherical contactor-I: Experimental apparatus and mathematical modeling. Chemical Engineering Science 61, 6571; Jamal, A., Meisen, A., Lim, C. J., 2006. Kinetics of carbon dioxide absorption and desorption in aqueous alkanolamine solutions using a novel hemispherical contactor-II: Experimental results and parameter estimation. Chemical Engineering Science 61, 6590). However, there has been limited study of the detailed analysis of the reaction pathway at an atomic level. The structure optimization, energy diagram, and transition-state exploration of the absorption and desorption processes are not clearly understood.